Thermal analysis comprises a series of powerful tech-niques for the characterization of the thermal, physical, mechanical and degradation properties of materials. One valuable application of thermal analysis is for the characterization of electronic materials and components, including printed circuit boards (PCB) and encapsulants.

Thermal analysis can help address the following key properties of electronic materials:

• SofteningtemperatureorTg

• Heatsofcure

• Degreeofcure

• Onsetofcure

• Maximumrateofcure

• Completionofcure

• Compositionalanalysis(thermosetandfiberglass)

• Onsetofdegradation

• Dimensionalstabilities

• Coefficientsofthermalexpansion

• Stressrelief

• Modulus(stiffness)properties

• Dampingorenergyabsorbingcharacteristics

Thermal Analysis

a p p l i c a t i o n n o t e

Characterization of Electronic Materials Using Thermal Analysis

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DSCcanalsobeusedtoexamineepoxymaterialsfor completion of cure based upon the occurrence of a residual curingexothermicpeak.Ifnopeakisobserved,thentheresin system is nearly completely cured or crosslinked. The followingDSCcurvesshowhowthecureexothermicpeakofanepoxyresinbecomessmallerasthedegreeofcureincreases.Athighlevelsofcure,theexothermicpeakcan no longer be detected.

Theheatofcuremeasuredonepoxymaterialcanbeused to assess the percent cure. The following simple equation provides this information:

% Cure = [DHuncured–DHsample]/ [DHuncured]•100%

Inthisequation,DHuncuredistheheatofcure(J/g)oftheunreacted resin or prepreg and DHsampleisthatoftheactual test specimen or material.

Inthemanufactureofprintedcircuitboardsorflip-chippackaging,theissueofcureoftheepoxymatrixandthedimensionalcharacteristicsoftheepoxy-basedmaterialsbecomescritical.Itisessentialtoensurethatthedegreeofcureobtainedbytheepoxymaterialsissufficientlyhighenough to ensure good stability and that the cure times are sufficiently rapid so that the processing time can be signifi-cantly reduced, thus providing a cost-savings.

DSC

Differentialscanningcalorimetry(DSC)measuresheatflowinto or from a sample under heating, cooling or isothermal conditions.ThefollowingDSCscanshowstheresultsobtainedfromanepoxy-fiberglassprepregusedtoproducePCB’s.

TheDSCresults,obtainedataheatingrateof20˚C/min,showthattheTgorsofteningoftheepoxyprepregmaterialoccursat60˚C.Withfurtherheating,theepoxyresinunder-goes cure and crosslinking and this is observed as a large exothermicpeakwithapeaktemperatureof160˚C.Thepeak temperature is equivalent to the point at which the resinachievesitsmaximumrateofcure.

DSCcanbeusedtoestablishthedegreeofcureachievedbyanepoxyresinoraPCBusingthefollowingtwomeans:

• Tg

• Residualheatofcure

Asanepoxyresingoestoahighercrosslinkdensityorgreaterdegreeofcure,itsTgincreasestoamaximumvalue,Tg(∞).ThefollowingfigureshowstheincreaseinTgofanepoxyresinwithregardstodegreeofcure.Thevaluesofthe glass transition temperature reach a plateau, which is Tg(∞),indicativeofnearlycompletecrosslinking.

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These results show that the autocatalyzed resin achieves its peakmaximumatseveralminutesintotheisothermalholdperiod. This reflects the time required to develop the internal catalysttoallowthemaincurereactiontoproceed.Incontrast,then-thorderepoxyresinhasitspeakmaximumattime=0. Thismeansthatthemaximumrateofcure,forn-thorderepoxyresins,reachesitsmaximumraterightatthestartofthe isothermal holding period.

Inordertobestestablishtheisothermalcureofepoxyresinsused for electronic purposes, and make the clear distinction between resins following n-th order or autocatalyzed reaction, itisabsolutelyessentialtohaveaDSCinstrumentwhichcan heattheresinveryquickly(i.e.,500˚C/min)andequilibrate rapidly at the desired isothermal temperature. The PerkinElmerpowercompensatedDSCmeetsbothofthesecriteriawiththeuseofverylowmass(1g)andseparatefurnacesforthesampleandreferencesides.ThisDSCisable to heat the uncured resin or prepreg sample from room temperature to the desired isothermal cure temperature at a controlledheatingrateof500˚C/min.Theinstrumentthenquickly equilibrates once the isothermal target temperature isreachedandthepeakmaximum,evenforn-thorder resins, is obtained.

Incontrast,thelargemassheatfluxDSCinstruments(furnacemassof200g),cannotheatorthermallyequilibratequickly.WiththesethermallysluggishDSCdevices,theonlywaythat isothermal cure studies can be performed is to pre-heat theDSCcelltothetargettemperature,manuallyopenthehotcell,andthendropthesampleintotheDSC,closethecellandthenallowtheDSCtoreequilibratebeforedataisacquired. This results in lost data when attempting to study isothermalcuresofepoxyresinsorprepregsbyheatfluxDSCinstruments.NovaluabledataislostwiththepowercompensatedDSC.

Forelectronicmaterials,thecureassociatedwithanepoxythermosetting resin may follow two distinctly different reaction mechanisms:

• N-thorder

• Autocatalyzed

Autocatalyzedepoxyresinsrequirethedevelopmentorbuild-up of an intermediate component which then catalyzes themaincuringreaction.N-thorderepoxyresinswillreactimmediately when the temperature is suitably high enough toallowthecuringreactiontoproceed.Itisimportanttoknowwhichofthetworeactionmechanismsanepoxyresinwill follow, as this will affect the cure and processing condi-tions.Generally,standardheatingDSCexperimentswillnotbeabletomakeobviousdistinctionsbetweenepoxyresinsas to whether they follow n-th order or autocatalyzed reaction kinetics.However,anisothermalDSCexperimentwillbeable to conclusively make this determination between n-th order or autocatalyzed kinetics.

ThefollowingDSCresultsrepresenttheisothermalcuring oftwoepoxyresinsoneofwhichfollowsann-thorder reaction and one which follows an autocatalyzed mechanism.

The high sensitivity PerkinElmer TGA instrument can be used to assess the very small weight losses associated with the epoxyprepregsusedtogeneratePCB’s.Itisdesiredtohavea low volatiles loss during cure to reduced the occurrence ofvoidinginPCB’saswellasencapsulants.ThefollowingresultsshowtheTGAmasslossat120˚Cforanepoxyresinunder isothermal conditions.

TheTGAresultsshowthattheepoxymateriallosesonly0.367%ofitsmass.Thelowgenerationofvolatilesindicates that the resin will have a low propensity for voiding during processing. The outstanding performance of the PerkinElmer TGA permits the high sensitivity measurements to be performed.

TMA

Thermomechanicalanalysis(TMA)measuresthedimensionalproperties(expansion,penetration,coefficientsofthermalexpansion)ofmaterialswhileheating,coolingorunder isothermalconditions.TMAisparticularlyusefulforthe electronics industry as it helps to ensure that individual components can be produced which have nearly identical coefficientsofthermalexpansion.Thishelpstoreducetheoccurrence of thermal stresses, which can adversely affect the integrity of the printed circuit board or flip-chip packaging. The build-up of significant thermal stresses and significantly shorten the lifetime of the component.

ThefigurebelowshowsTMAresultsintheexpansionmodeobtainedonanepoxyPCB.Theplotshowsthe%length(expansion)versustemperature.Thesamplewasanalyzedasreceived(1stheat),cooledbacktothestartingtemperature,andthenreheated(2ndheat).

Duringthe1stheatingsegment,thePCBexhibitsstressreliefasthetemperaturereachestheTgoftheepoxymatrix.The stresses are a result of processing and are frozen into the material as it cools down below its Tg. The stress relief ismanifestedasanon-linearresponseinthesample’sexpansionresponseaboveTg.Duringthefirstheating,thestress factors are dissipated and are no longer observed during the second heating segment. A simple change in theexpansionrateisobservedduringthe2ndheatandtheonsettemperaturereflectstheTgoftheepoxymatrix.ForthisPCB,theTgisfoundtobe121.7˚C.

FromtheTMAexpansionresults,thecoefficientofthermalexpansion(CTE)canbeassessedbelowandaboveTg.Thecoefficientofthermalexpansionisdefinedas:

CTE = DL/[DT•Lo]

where DL/DTisthemeasuredchangeinexpansionoveratemperatureinterval(mm/˚C)andLoisthesample’soriginalthickness (mm).

TGA

Thermogravimetric analysis (TGA) is used to measure sample mass losses and decomposition temperatures under heating orisothermalconditions.Tocontroland/orreducetheexpansionratesofPCB’sorencapsulants,inertmaterials,suchasfiberglassorsilica,isaddedtotheepoxyresins.Itisessentialtoknowthelevelofepoxyandfillerinaprepregor compound as this compositional information will have a major effect on the end-use characteristics of the material.

ThefollowingTGAresultsarethoseobtainedonanepoxy–fiberglass prepreg material used to generate printed circuit boards.Thesamplewasanalyzedinnitrogenupto700˚Candtheninoxygentoburnoffanycarbonchartoleavebehind the inert residue (fiberglass).

Fromtheseresults,thetotalepoxycontent(weightlossesinnitrogenandoxygen)arefoundtobe67.0%andtheinertfiberglassresiduecontentis33.0%.Theonsettemperatureofepoxydegradationgivesanindicationastoitsthermalstability.Higherdecompositiontemperaturesgenerally translate into better stabilities.

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IntheTMAresultsfortheepoxyfiberglassPCB,itwasfoundthatthevalueoftheCTEbelowTgwas5.0510-5/˚whileabove the glass transition, it was significantly higher with a valueof2.7110-4/˚.ThevaluesoftheCTEarecriticalfor electronic applications to ensure that the PCB or the components in the PCB or in flip-chip packaging have proper and matching expansionlevels.IfthereisamismatchbetweenCTE’s,thermalstresses can result during operation and this can significantly decrease the integrity of the electronic component.

DMA

Dynamicmechanicalanalysis(DMA)measurestheviscoelasticproperties of materials as a function of temperature (heating, coolingorisothermal)andasafunctionoffrequency.DMAisusedtomeasureamaterial’sstoragemodulus(stiffness)andloss modulus (energy absorption or damping) over a temperature range.Intermsofassessingasample’sTgandthecompleteness ofcure,DMAisthemostsensitiveofthethermalanalysistechniques.

For electronic components, it is desired to know the values of thestoragemodulus,E’,overthetemperaturerangeofoperation.ForPCB’sandunderfillmaterials,itisgenerallydesiredtohavea high value of the modulus as this helps to effectively distribute thermal stresses.

ShownintheexamplebelowaretheDMAresultsobtainedonanepoxy-fiberglassPCB.Theplotshowsthestoragemodulus(E’)andtandelta(E”/E’)asafunctionofsampletemperatureatafrequencyof1.00Hzusingthe3-pointbendingmodeofdeformation.

Theglasstransitionoftheepoxymatrixisobservedasalargedrop in the storage modulus and a peak in the tan delta responseat141˚C.TheseresultsindicatethatthePCBisfullycuredasnoincreaseisobservedintheE’responseaboveTg.Iftheepoxymatrixwasslightlyunder-cured,anincreasewould be obtained in the storage modulus response above Tg reflective of additional or residual crosslinking.

The residual crosslinking of an unacceptable PCB is displayed inthefigurebelowshowingDMAstoragemodulusresultson‘good’and‘bad’epoxy-fiberglasscomposites.

The rejected PCB was slightly uncured and had a significantly lower Tg than the acceptable, more crosslinked material. Also,theundercuredepoxy-fiberglasscompositeexhibitedanincrease in its modulus response above Tg, which indicates additional crosslinking.

Summary

Thermalanalysiscomprisesaseriesoftechniques(DSC,TGA,TMAandDMA)forthecompletecharacterizationofelectronicmaterials. Valuable information can be obtained on such properties as: softening or Tg, completeness of cure, degree of cure, rate of cure, compositional analysis, degradation temperaturesforstabilityassessment,expansivities,coefficientsofthermalexpansion,modulus(stiffness)anddampingproperties.

PerkinElmer offers a complete line of state-of-the-art thermal analysisinstruments(DSC,TGA,TMAandDMA)fortheanalysisand characterization of the critical thermal, physical, degradation and mechanical properties of electronic materials. The results are highly useful for research and development as well as quality assurance applications.